Fluid powered reciprocating piston motors have long been in use, particularly in heavy truck and bus windshield wiper drivers. Previously, fluid powered motors have had the fluid flow controlled by a simple gate valve which was operably connected to a mechanical switch. When the gate valve is switched to an on position, fluid flow to the park port of the motor is cut off. This causes a shiftable spool valve to shift into a run position which aligns the various fluid ports of the motor. Fluid flow through the manifold and motor ports causes the reciprocating movement of the piston which is typically connected to a wiper armby rack and pinion gears.
A typical fluid powered motor as described above is shown and described in U.S. Pat. No. 4,632,013, which is also owned by the assignee of this invention, and is incorporated herein by reference. Typically, the motor is powered by pressurized air, but any compressible fluid can be adapted to power the motor. Motors constructed according to the teachings of the '013 patent are highly dependable performers with regard to both accuracy and durability.
Such fluid-powered motors have several shortcomings however. Because of the simple nature of the control valve, these motors are generally operated at only low and high duty cycle speeds, or in a continuous intermittent mode. Because the control valve is normally shifted only between full open and full closed positions, the reciprocating piston and connected wiper arm are normally either stationary or operating at constant speed.
A second drawback is the requirement of two separate fluid lines and control valves to operate the motor in low and high speeds. Automatic intermittent duty cycles are not passable in a mechanically controlled fluid powered motor.
A third drawback is the problem of whipstroke, which occurs during the first duty cycle of a typical motor. Whipstroke results from the lack of a fluid cushion in the cylinder chamber opposite the chamber which receives the first delivery of pressurized fluid after the motor is switched on.